During their first year, infants learn characteristics of the sound-pattern (phonology) of the language they hear around them. Infants also learn the forms of many words. The goal of the present research is to provide an empirically supported account of how the knowledge gained in infancy supports the language acquisition process. The proposed studies include perceptual experiments with infants and young children, acoustic measurements and annotation of infant-directed speech, and computational modeling of corpora of speech directed to infants. Several studies test children's perception of certain speech sound distinctions under varying conditions and children's learning of phonological information in new words. These studies take advantage of recent methodological advances in research on early word learning and speech perception, including eyetracking techniques. The computational modeling work uses what is known about infant perception to make estimates of infants'word-form knowledge in five languages. This word-form knowledge is a determinant of children's biases in how they find words in continuous speech, and contributes to early generalizations about the nature of words in the language. The effort to properly characterize young children's phonological knowledge and how it arises in infancy is relevant to the developmental timing of intervention for hearing deficits;it informs understanding of early receptive and productive vocabulary development;and it sheds light on current debates concerning the skills and mental representations that underlie successful performance in learning to read.